U.S. patent application number 17/474286 was filed with the patent office on 2021-12-30 for electrochemical oxidation of aromatic aldehydes in acidic media.
The applicant listed for this patent is WISCONSIN ALUMNI RESEARCH FOUNDATION. Invention is credited to Kyoung-Shin Choi, Stephen Riley Kubota.
Application Number | 20210404071 17/474286 |
Document ID | / |
Family ID | 1000005839902 |
Filed Date | 2021-12-30 |
United States Patent
Application |
20210404071 |
Kind Code |
A1 |
Choi; Kyoung-Shin ; et
al. |
December 30, 2021 |
ELECTROCHEMICAL OXIDATION OF AROMATIC ALDEHYDES IN ACIDIC MEDIA
Abstract
Methods for electrochemically oxidizing aromatic aldehydes, such
as furfural and furfural derivatives, to carboxylic acids in acidic
solutions are provided. Also provided are electrochemical cells for
carrying out the oxidation reactions. The electrochemical
oxidations may be conducted in aqueous media at ambient pressure
and mild temperatures.
Inventors: |
Choi; Kyoung-Shin;
(Fitchburg, WI) ; Kubota; Stephen Riley; (Madison,
WI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
WISCONSIN ALUMNI RESEARCH FOUNDATION |
Madison |
WI |
US |
|
|
Family ID: |
1000005839902 |
Appl. No.: |
17/474286 |
Filed: |
September 14, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
15727992 |
Oct 9, 2017 |
11142833 |
|
|
17474286 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25B 11/04 20130101;
C25B 11/054 20210101; C25B 3/23 20210101 |
International
Class: |
C25B 3/23 20060101
C25B003/23; C25B 11/04 20060101 C25B011/04; C25B 11/054 20060101
C25B011/054 |
Goverment Interests
REFERENCE TO GOVERNMENT RIGHTS
[0002] This invention was made with government support under
DMR-1121288 awarded by the National Science Foundation. The
government has certain rights in the invention.
Claims
1. A method for the electrochemical oxidation of furfural in an
electrochemical cell comprising: an anode comprising manganese
oxide in an anode electrolyte solution; and a cathode in a cathode
electrolyte solution, wherein the anode electrolyte solution
comprises the furfural and has a pH lower than 7, the method
comprising: applying an anode potential to the anode that induces
the electrochemical oxidation of the furfural to maleic acid.
2. The method of claim 1, wherein the maleic acid is formed with a
product yield of at least 20%.
3. The method of claim 1, wherein the maleic acid is formed with a
product yield of at least 30%.
4. The method of claim 1, wherein the maleic acid is formed with a
product yield in the range from 20% to 40%.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a divisional of U.S. patent
application Ser. No. 15/727,992 that was filed Oct. 9, 2017, the
entire contents of which are incorporated herein by reference.
BACKGROUND
[0003] Biomass is a promising sustainable material source for the
manufacture of key building block chemicals as well as fuels to
reduce or eliminate dependence on fossil fuels.
5-Hydroxymethyfurfural (HMF), which can be derived from cellulosic
biomass, has generated a considerable interest as a platform
molecule to synthesize industrially and commercially desirable
products. For example, 2,5-furandicarboxylic acid (FDCA), one of
the oxidation products of HMF, can serve as a monomer in the
synthesis of a variety of polymeric materials. In particular, it is
known to be an excellent replacement for terephthalic acid in many
polyesters such as polyethylene terephthalate (PET).
[0004] Traditional HMF oxidation to FDCA typically relies on
chemical oxidants or noble metal catalysts in high temperature and
O.sub.2 pressure reactions. Aqueous electrochemical oxidation is a
very promising alternative, because reactions are driven by an
applied potential, removing the need for chemical oxidants and high
pressure O.sub.2.
[0005] Studies have shown that efficient and selective
electrochemical oxidation of HMF to FDCA is indeed possible;
however these reactions operate at elevated pH where FDCA is
soluble. In an industrial application, lowering the pH after every
reaction to precipitate the FDCA generates a tremendous amount of
salt as waste and adds costs associated with purchasing the
necessary acid/base and removing excess salt from solution.
SUMMARY
[0006] Methods and electrochemical cells for electrochemically
oxidizing aromatic aldehydes to carboxylic acids in acidic
solutions are provided.
[0007] One embodiment of a method for the electrochemical oxidation
of an aromatic aldehyde is carried out in an electrochemical cell
that includes: an anode that is active for the electrochemical
oxidation of the aromatic aldehyde in an anode electrolyte
solution; and a cathode in a cathode electrolyte solution. The
anode electrolyte solution includes the aromatic aldehyde and has a
pH lower than 7. The method entails: applying an anodic potential
to the anode that induces the electrochemical oxidation of the
aromatic aldehyde to a carboxylic acid. In embodiments of the
method wherein the aromatic aldehyde is 5-hydroxymethylfurfural and
the carboxylic acid is 2,5-furandicarboxylic acid, the
2,5-furandicarboxylic acid can be produced with a yield of at least
10%.
[0008] Other principal features and advantages of the invention
will become apparent to those skilled in the art upon review of the
following drawings, the detailed description, and the appended
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] Illustrative embodiments of the invention will hereafter be
described with reference to the accompanying drawings, wherein like
numerals denote like elements.
[0010] FIG. 1 depicts possible reaction schemes for the oxidation
of HMF to FDCA.
[0011] FIG. 2A shows a scanning electron microscope (SEM) image of
a MnO.sub.x film, as deposited. FIG. 2B shows an SEM image of a
MnO.sub.x film after annealing.
[0012] FIG. 3 depicts linear sweep voltammetry (LSV) curves of a
MnO.sub.x electrode obtained in a pH 1H.sub.2SO.sub.4 solution, as
described in Example 1: without added substrates; with 10 mM HMF;
10 mM DFF; and 10 mM FFCA at a scan rate of 5 mV s.sup.-1.
[0013] FIG. 4 shows possible reaction schemes for the oxidation of
HMF to maleic acid.
[0014] FIG. 5 shows possible reaction schemes for the oxidation of
furfural to maleic acid.
DETAILED DESCRIPTION
[0015] Methods for electrochemically oxidizing aromatic aldehydes,
such as furfural and furfural derivatives, to carboxylic acids, for
example, dicarboxylic acids, in acidic solutions are provided. Also
provided are electrochemical cells for carrying out the oxidation
reactions. The electrochemical oxidations may be conducted in
aqueous media at ambient pressure (about 1 atm) and mild
temperatures (0.degree. C.<T<100.degree. C.).
[0016] In some embodiments of the methods, the use of an acidic
solution is advantageous because it results in the precipitation of
the carboxylic acid, which facilitates its separation from the
solution. In some embodiments of the methods, the use of an acidic
solution is advantageous because it results in the formation of
products that would not form, or that would form only in minimal
amounts, in neutral or basic solutions. By way of illustration,
embodiments of the methods can be carried out in anode electrolyte
solutions having a pH of less than 7. This includes anode
electrolyte solutions having a pH of no higher than 5, anode
electrolyte solutions having a pH of no higher than 4, anode
electrolyte solutions having a pH of no higher than 3, anode
electrolyte solutions having a pH of no higher than 2, and anode
electrolyte solutions having a pH of no higher than 1. For example,
the electrochemical oxidations can be carried out in anode
electrolyte solutions having a pH in the range from 0.1 to 6.
[0017] The electrochemical methods and cells use anodes that are
active for the oxidation of the aromatic aldehydes. For the
purposes of this disclosure, an anode is active for the oxidation
of an aromatic aldehyde if at least a portion of the anodic current
is used for the electrochemical oxidation of the aromatic aldehyde
during the operation of the cell--even if some of the current
generated at the anode is associated with the electrochemical
oxidation of water.
[0018] Some embodiments of the anodes are more active for the
oxidation of the aromatic aldehydes than they are for the oxidation
of water in the acidic solution in which the oxidation is carried
out. For these anodes it is possible to oxidize the aromatic
aldehyde without oxidizing water by operating the electrochemical
cell at a voltage that allows only for the oxidation of the
aromatic aldehyde and its oxidation intermediates.
[0019] Examples of anode materials that can be used in the methods
and cells include, metal oxides, such as MnO.sub.x, where the x
indicates that the oxidation state of Mn in the compound can be 3+,
4+, or a mix of 3+ and 4+. Other anode materials include oxides,
such as PbO.sub.2, CeO.sub.2, WO.sub.3, TiO.sub.2, Ta.sub.2O.sub.5,
Nb.sub.2O.sub.5, IrO.sub.2, and RuO.sub.2, metals, such as Au, Pd,
and Pt, and carbon-based electrodes (e.g., graphitic carbon, glassy
carbon, and the like).
[0020] The aromatic aldehydes have an aromatic ring with at least
one aldehyde group-containing substituent. The aromatic rings can
be homocyclic or heterocyclic rings. Other types of functional
groups may also be present on the aromatic ring--in addition to
aldehyde groups. For example, the aromatic aldehydes can include
one or more alcohol groups and/or one or more carboxylic acid
groups on the aromatic ring. Furfural (C.sub.5H.sub.4O.sub.2) and
furfural derivatives are examples of aromatic aldehydes that can be
electrochemically oxidized. As used herein, a furfural derivative
is a compound that has a furan ring with at least one aldehyde
substituent and one or more additional ring substituents. Examples
of furfural derivatives include HMF, 2,5-diformylfuran (DFF), and
2-formyl-5-furancarboxylic acid (FFCA).
[0021] HMF can be oxidized to form the aromatic dicarboxylic acid,
FDCA, in an oxygen-donating, acidic electrolyte solution, such as
water, as illustrated in Example 1. This oxidation involves the
electrochemical oxidation of the aldehyde group of HMF to a
carboxylic acid and also the electrochemical oxidation of the
alcohol group of HMF to a carboxylic acid, which can occur under
the same oxidation conditions. Two possible pathways to form FDCA
are shown in FIG. 1. One pathway forms DFF as the first
intermediate by the oxidation of the alcohol group of HMF, while
the other pathway forms 5-hydroxymethyl-2-furancarboxylic acid
(HMFCA) as the first intermediate by the oxidation of the aldehyde
group of HMF. In an anode electrolyte that serves as an oxygen
donor, both DFF and HMFCA are further oxidized to form
5-formyl-2-furancarboxylic acid (FFCA) and then FDCA.
[0022] Among the various approaches used to oxidize HMF to FDCA,
electrochemical oxidation of HMF in aqueous media can provide
several distinct advantages. First, as the oxidation is driven by
the electrochemical potential applied to the electrode, the use of
chemical oxidants that may be environmentally harmful can be
completely eliminated. Since water serves as an oxygen donor for
the formation of the carboxylic acid groups from the alcohol and
aldehyde groups, no chemicals other than HMF and water are
necessary to form FDCA. Second, electrochemical oxidation can be
effectively performed at ambient pressure and mild temperatures.
Third, since electrochemical oxidation is coupled with
electrochemical reduction, electrons obtained at the anode from HMF
oxidation can be simultaneously used for a valuable reduction
reaction at the cathode, which can significantly increase the value
of the electrochemical approach. Additionally, because FDCA is
insoluble near room temperature and low pH (e.g., .ltoreq.3),
carrying out the electrochemical oxidation of HMF in a sufficiently
acidic anode electrolyte solution has the further advantage of
facilitating the recovery of the FDCA through precipitation. This
can be accomplished by carrying out the electrochemical oxidation
of HMF in an acidic solution at a temperature at which the FDCA
precipitates out of the anode electrolyte solution as it is formed.
Alternatively, the electrochemical oxidation of HMF in an acidic
solution can be carried out at a temperature at which the FDCA
remains soluble in the anode electrolyte solution. Then, once the
electrochemical oxidation is complete, the temperature of the anode
electrolyte solution can be cooled to a temperature at which the
FDCA becomes insoluble and precipitates out of the anode
electrolyte solution. The precipitated FDCA can then easily be
separated from the solution.
[0023] HMF can also be oxidized to form maleic acid in an
oxygen-donating, acidic electrolyte solution, as illustrated in
Example 2. Possible reaction schemes for the oxidation of HMF to
maleic acid are shown in FIG. 4, where the byproducts can be
CO.sub.2 and/or HCOOH. Furfural is another molecule that can be
oxidized to form maleic acid in an oxygen-donating, acidic
electrolyte solution, as illustrated in Example 3. Possible
reaction schemes for the oxidation of furfural to maleic acid are
shown in FIG. 5. The use of an acidic anode electrolyte solution
may facilitate the formation of maleic acid via the electrochemical
oxidation of furfural and furfural derivatives, as these compounds
would not form, or would form only in minimal amounts, in neutral
or basic anode electrolyte solutions. Maleic acid is a useful
product, since it can serve as an intermediate in the production of
succinic acid, a commercially valuable chemical.
[0024] In some embodiments of the electrochemical oxidation
methods, HMF is oxidized in an acidic anode electrolyte solution to
form a mixture of FDCA and maleic acid. These two products are
readily separated since the FDCA precipitates out of the solution
at or near room temperature, while the maleic acid remains
soluble.
[0025] One embodiment of an electrochemical cell for carrying out
the electrochemical oxidations comprises an anode in an acidic
anode electrolyte solution comprising a solvent, typically water,
and an aromatic aldehyde. An acid, such as sulfuric acid, can be
added to the anode electrolyte solution to provide an acidic pH.
The anode electrolyte solutions may further include a buffer to
maintain a given pH. A cathode in a cathode electrolyte solution is
in electrical communication with the anode. The solvent of the
anode and cathode electrolyte solutions may the same or different.
To operate the electrochemical cell, a voltage source is used to
apply an anodic potential to the anode and a potential difference
is created between the anode and the cathode. Driven by this
potential difference, electrons flow from the anode to the cathode
through an external wire. The electrons at the surface of the
cathode undergo reduction reactions with species contained in the
cathode electrolyte solution, while oxidation reactions occur at
the anode.
[0026] In some embodiments of the electrochemical cells, the
cathode reaction is the reduction of water to H.sub.2. However,
other cathode reactions are possible, including the reduction of
carbon dioxide to form carbon based fuels, such as methanol or
methane, or the reduction of organic molecules to form more
valuable organic chemicals. A variety of materials can be used for
the cathode, depending on the reduction reaction that is being
carried out. For example, if the reduction of water to H.sub.2 is
the cathode reaction, platinum, which is catalytic for hydrogen
evolution, can be used as the cathode.
[0027] The electrochemical oxidation of the aromatic aldehydes can
be carried out with substantial product yields. For example, HMF
can be oxidized to FDCA with a product yield of at least 30%, at
least 40%, or at least 50% or to maleic acid with a product yield
of at least 10%, at least 20%, or at least 30%. Furfural can be
oxidized to maleic acid with a product yield of at least 20%, at
least 30%, or at least 40%. The product yield (%) is calculated
using the following equation:
Yield .times. .times. of .times. .times. product .function. ( % ) =
mol . .times. of .times. .times. product .times. .times. formed mol
. .times. of .times. .times. initial .times. .times. aromatic
.times. .times. aldehyde .times. 1 .times. 0 .times. 0 .times. % .
##EQU00001##
EXAMPLES
Example 1: Electrochemical Oxidation of HMF to FDCA
[0028] This example illustrates the electrochemical oxidation of
HMF to FDCA in acidic media using MnO.sub.x as an illustrative
anode material. This approach eliminates the need to vary the pH of
the reaction solution in order to recover the FDCA.
[0029] Manganese Oxide (MnO.sub.x) as a Anode for HMF Oxidation
[0030] The MnO.sub.x electrodes used in this example were prepared
by electrodeposition. (The notation of MnO.sub.x is used because
the film contains a mixture of Mn.sup.3+ and Mn.sup.4+ ions in an
ill-defined ratio and, therefore, the amount of oxygen present in
the compound was not accurately determined.) An aqueous solution
composed of 50 mM MnSO.sub.4 and 100 mM Na.sub.2SO.sub.4 was used
as a plating solution. Anodic electrodeposition was carried out in
an undivided three-electrode cell. Glass coated with fluoride doped
tin oxide (FTO) and Pt were used as the working electrode (WE) and
counter electrode (CE), respectively. An Ag/AgCl (4 M KCl)
electrode was used as the reference electrode (RE). MnO.sub.x was
anodically deposited by applying 0.9 V to the WE in the plating
solution kept at 60.degree. C. while passing 0.5 C/cm.sup.2. The
anodic bias oxidized Mn.sup.2+ ions in the plating solution to
Mn.sup.4+ ions, which are no longer soluble and precipitate as a
MnO.sub.x film on the WE. The as-deposited films were washed with
deionized (DI) water, dried in a stream of air, and then annealed
at 400.degree. C. for 2 hours, with a ramp rate of 2.degree.
C./min. The annealed film, as well as the as-deposited film, was
X-ray amorphous. SEM images of the as-deposited and annealed
samples showing their surface morphologies are displayed in FIGS.
2A and 2B, respectively.
[0031] The activity of MnO.sub.x for HMF oxidation to FDCA was
first examined using LSV in a H.sub.2SO.sub.4 (pH 1) solution with
and without 10 mM HMF and the same concentration of HMF oxidation
intermediates, DFF and FFCA (FIG. 3). The major competing reaction
for HMF oxidation in aqueous conditions is the electrochemical
oxidation of water, which can be observed in the LSV obtained
without HMF and its oxidation intermediates.
[0032] After the addition of HMF, DFF, or FFCA, the oxidation
current onset potential was commonly shifted to the less positive
potential. This demonstrates that it is possible to oxidize HMF all
the way to FDCA without oxidizing water by choosing a potential
that allows only for the oxidation of HMF and its oxidation
intermediates.
[0033] Constant potential oxidation of HMF to FDCA was carried out
at 1.6 V vs. RHE (1.34 V vs. Ag/AgCl) at 60.degree. C. using a cell
divided with a glass frit. The WE compartment (anolyte) contained
15 mL of a H.sub.2SO.sub.4 solution (pH 1) containing 20 mM HMF,
while the CE compartment (catholyte) contained 15 mL of a
H.sub.2SO.sub.4 solution (pH 1) solution. The anode, cathode, and
the overall reactions are summarized below.
HMF+2H.sub.2O.fwdarw.FDCA+6H.sup.++6e.sup.- Anode reaction:
6H.sup.++6e.sup.-.fwdarw.3H.sub.2 Cathode reaction:
HMF+2H.sub.2O.fwdarw.FDCA+3H.sub.2 Overall reaction:
[0034] The elevated temperature was used to improve the kinetics of
HMF oxidation using MnO.sub.x. Another advantage of using the
elevated temperature is that the solubility of FDCA is considerably
increased compared to that at room temperature (RT;
.about.23.degree. C.). As a result, FDCA precipitation did not
occur during electrochemical oxidation until the electrolysis was
completed and the solution was cooled down to RT. This can be
favorable, as the precipitation of FDCA on the electrode surface
during electrolysis may hinder electrochemical oxidation. By using
an acidic medium, the separation of FDCA was enabled by altering
temperature instead of altering pH, which significantly simplifies
the separation process.
[0035] The concentration changes of HMF and its oxidation products
in the anolyte were determined using high-performance liquid
chromatography (HPLC). The yields (%) of the oxidation products
were calculated using the following equation:
Yield .times. .times. of .times. .times. product .function. ( % ) =
mol . .times. of .times. .times. product .times. .times. formed mol
. .times. of .times. .times. initial .times. .times. HMF .times.
100 .times. % . ##EQU00002##
[0036] The stoichiometric amount of charge required to completely
convert 15 mL of 20 mM HMF solution to FDCA is 174 C. The product
analysis obtained at 250 C showed that FDCA yield was 54%.
[0037] The major byproduct was maleic acid, obtained by the
reaction shown in FIG. 4. The formation of maleic acid does not
affect the separation process of FDCA, as it is highly soluble in
acidic pH. Maleic acid can also serve as an intermediate to produce
succinic acid that, along with FDCA, is a value-added chemical that
can be derived from biomass.
Example 2: Electrochemical Oxidation of HMF to Maleic Acid
[0038] This example illustrates the electrochemical oxidation of
HMF to maleic acid in acidic media using lead oxide (PbO.sub.2) as
an illustrative anode material.
[0039] The PbO.sub.2 electrodes used in this example were prepared
by electrodeposition. An aqueous solution composed of 50 mM
Pb(NO.sub.3).sub.2 lowered to a pH of 1 with nitric acid was used
as the plating solution. Anodic electrodeposition was carried out
in an undivided three-electrode cell. FTO and Pt were used as the
WE and CE, respectively. An Ag/AgCl (4 M KCl) electrode was used as
the RE. PbO.sub.2 was anodically deposited by applying 2 V to the
WE while passing 0.25 C/cm.sup.2. Under the applied anodic bias,
soluble Pb.sup.2+ species were oxidized to insoluble Pb.sup.4+
species, which deposited onto the WE as PbO.sub.2. After
deposition, films were rinsed with DI water, dried in a stream of
air and then used as-deposited.
[0040] Constant potential oxidation of HMF was carried out at 2.0 V
vs. RHE (1.74 V vs. Ag/AgCl) at 60.degree. C. using a cell divided
with a glass frit. The WE compartment contained 15 mL of a
H.sub.2SO.sub.4 solution (pH 1) containing 20 mM HMF, while the CE
compartment contained 15 mL of a H.sub.2SO.sub.4 solution (pH 1).
The stoichiometric amount of charge required to completely convert
15 mL of 20 mM HMF solution to maleic acid, assuming that only
CO.sub.2 is formed as a byproduct, is 347 C. Products were analyzed
via HPLC. At 200 C of charge passed the maleic acid yield was
35.5%. Maleic acid was the major, identifiable HMF oxidation
product. The formation of FDCA was negligible.
Example 3: Electrochemical Oxidation of Furfural to Maleic Acid
[0041] This example illustrates the electrochemical oxidation of
furfural to maleic acid in acidic media using manganese oxide
(MnO.sub.x) as an illustrative anode material.
[0042] The MnO.sub.x electrodes used in this example were prepared
as in Example 1. Constant potential oxidation of furfural was
carried out at 1.7 V vs. RHE (1.44 V vs. Ag/AgCl) at 60.degree. C.
using a cell divided with a glass frit. The WE compartment
contained 15 mL of a H.sub.2SO.sub.4 solution (pH 1) containing 10
mM furfural, while the CE compartment contained 15 mL of a
H.sub.2SO.sub.4 solution (pH 1). Products were analyzed via nuclear
magnetic resonance (NMR) because the HPLC setup used in example 1
and 2 could not provide accurate quantification of furfural. The
stoichiometric amount of charge required to completely convert 15
mL of 10 mM furfural solution to maleic acid, assuming that only
CO.sub.2 is formed as a byproduct, is 116 C. At 179 C of charge
passed the maleic acid yield was 47%. Maleic acid was the major,
identifiable furfural oxidation product.
[0043] The word "illustrative" is used herein to mean serving as an
example, instance, or illustration. Any aspect or design described
herein as "illustrative" is not necessarily to be construed as
preferred or advantageous over other aspects or designs. Further,
for the purposes of this disclosure and unless otherwise specified,
"a" or "an" means "one or more."
[0044] The foregoing description of illustrative embodiments of the
invention has been presented for purposes of illustration and of
description. It is not intended to be exhaustive or to limit the
invention to the precise form disclosed, and modifications and
variations are possible in light of the above teachings or may be
acquired from practice of the invention. The embodiments were
chosen and described in order to explain the principles of the
invention and as practical applications of the invention to enable
one skilled in the art to utilize the invention in various
embodiments and with various modifications as suited to the
particular use contemplated. It is intended that the scope of the
invention be defined by the claims appended hereto and their
equivalents.
* * * * *